The present invention relates to current sources for loads of a considerable range of impedance values which include an inductive component and, more particularly, to a current source for such loads through which the direction of current flow must alternate.
Current sources which provide a relatively constant current, at least for successive periods of time, are needed in many applications including ones which require establishing relatively constant magnetic fields for such periods of time. Such currents can be difficult to keep constant for such successive periods of time if the current flow direction must be reversed between each of these periods and they are of relatively short duration. This is especially so where the periods of desired current constancy are to occur periodically, with a flow direction reversal between each, and at a frequency which is significant with respect to various time constants occurring in the system in which this current is provided.
One such system in which these conditions arise is electromagnetic flowmeters. In these systems, a magnetic field is established across a metering tube through which is flowing a liquid, or liquid-like, substance which exhibits at least some electrical conductivity. Conductive media flowing through a magnetic field lead, in accord with electromagnetic theory, to the establishment of an electromotive force or voltage perpendicular to both the flow direction and the magnetic field direction, a voltage which is proportional to the average velocity of flowing fluid. The provision of electrodes at the locations in which the voltage is primarily developed permits obtaining a signal which is linearly representative of the velocity of the liquid from which its liquid flow can be determined.
However, if the magnetic field is constant in magnitude and direction, the resulting constant polarity voltage portion due to the fluid flow induced signal cannot be separated from the portion due to the electro- chemical potential of the flowing fluid and the sensing electrodes together. Further, the resulting direct signal current established in the flowing fluid, transverse to the direction of its flow, can lead to polarizing the two sensing electrodes over time thereby adversely affecting the output voltage signal representation of flow. To avoid this result, the magnetic field is usually applied to the flow tube alternately in opposite directions to balance out such transverse current flows and so avoid a net polarization of the sensing electrodes.
The frequency of reversals, or alternations of direction, of the magnetic field through the flow tube has a bearing on performance of the electromagnetic flow measurement system. On the one hand, a higher frequency of reversal will further separate that frequency from the noise in the signal taken from the sensing electrodes that is of a type which can be described as 1/f noise. On the other hand, the signal transmission leads carrying the signal from the sensing electrodes acts as a transmission line, and can be relatively long if the data capture site is a substantial distance from the flow measurement site. In these circumstances, the transmission line distributed capacitance and the resistance of the fluid will have the electrical characteristics of a low-pass filter. Thus, at some point, increasing the frequency of reversals forming the basis of periodic variation in the electrical signals from the sensing electrodes, will lead to reduced amplitudes in such signals obtained from these electrodes at the data capture location because of the filtering action of the connecting signal line.
In electromagnetic flowmeter systems, there is a desire to have a constant current flowing in magnetic field coils during the times of obtaining values representing flow from the sensed signal in any period of the alternation of the applied magnetic field. If the magnetic field is constant during such obtaining of sense signal values, there will not be much inductive pickup occurring in system portions therearound including in the apparatus used in obtaining the flow representation signal, or sense signal, from the sensing electrodes. Thus, less noise or offset will be present in this sense signal.
However, difficulty arises as the frequency of reversals of the applied magnetic field increases. The magnetic field strength B in the fluid results from various currents flowing in the electromagnetic flowmeter metering system including (i) the current applied to the coils from a current source to provide the desired magnetic field, and (ii) the resulting eddy currents which are induced to flow in the conductive metering tube and the magnetic materials in the magnetic return circuit. The magnetic field strength B in the flowing fluid thus approaches a steady value exponentially after a current reversal, due to the shielding effects of the eddy currents. Thus, changes in the B field in the flowing fluid material lag the changes in the applied current giving rise to them, and do so for a greater fraction of each of the periods of field direction alternation the more frequently the reversals occur, i.e. the shorter the period.
Hence, the amount of time that the B field is constant in each period becomes smaller and smaller as the frequency of the reversals increases thereby leading to less and less time to acquire a sensed value in the sense signal from the sense electrodes which is substantially free of inductive noise as a result of the inductive pickup noise having had time to subside. At some point of increasing frequency, this leads to an increase in the noise in the signal provided by the sense electrodes, and can lead to a substantial decrease in the magnitude of the sense signal because the B field remains substantially reduced by the eddy current in each period.
This latter condition is worsened if the applied current provided in the electromagnetic flowmeter system also has a relatively long rise time after each reversal of its direction of current flow as required to provide the corresponding reversal of the applied B field. After a current direction reversal thereof, delay in reaching a desired constant current value leads to shorter times also for the resulting B field generated by it and the induced eddy currents to come to a constant value in any period after a field direction alternation. Thus, there is a desire to shorten the rise time of the current used to provide the B field in an electromagnetic flowmeter after a flow direction reversal.